Twisted pair public telephone lines are increasingly being used to carry relatively high-speed signals instead of, or in addition to, telephone signals. Examples of such signals are ADSL (asymmetric digital subscriber line), HDSL (High Density Subscriber Line, T1 (1.544 Mb/s), and ISDN signals. There is a growing demand for increasing use of telephone lines for high speed remote access to computer networks, and there have been various proposals to address this demand, including using DOV (data over voice) systems to communicate signals via telephone lines at frequencies above the voice-band.
The provision in the public telephone network of varied services using such diverse communications systems imposes a requirement that different and similar systems not interfere with one another. A predominant limiting effect in this respect is NEXT (near end crosstalk) between wire pairs within multiple-pair cable binder groups or between wire pairs within adjacent binder groups.
Allocations of wire pairs within telephone cables in accordance with service requests have typically resulted in a random distribution of pair utilization with few precise records of actual configurations. In addition, due to the nature of pair twisting in cables, and where cable branching and splicing occurs, a wire pair can be in close proximity to different other pairs over different parts of its length. At a telephone C.O. (central office), pairs in close proximity may be carrying diverse types of service using various modulation schemes, with considerable differences in signal levels (and receiver sensitivities) especially for pairs of considerably different lengths.
Statistical data has been developed that can be used to estimate crosstalk between services using different pairs of multi-pair telephone cables, for example in terms of BER (bit error rate) based on power spectral density (PSD, for example measured in milliwatts per Hertz expressed in decibels, or dBm/Hz) overlap between the services. However, this statistical data is of limited use in practice in the provision of a new service using equipment connected to a specific wire pair, in view of factors such as those discussed in the preceding paragraph.
It is therefore a significant concern of telephone companies that the signals and operation of existing systems may be adversely affected, especially as a result of NEXT, by the deployment of new equipment, particularly digital signal transmission equipment. This concern is increased in accordance with the extent to which such equipment is likely to be deployed, and hence particularly applies to equipment that may be used in very large numbers for remote access to computer networks. New equipment can be designed in a manner largely to avoid interference with other systems in accordance with the statistical data, but this imposes undesirable constraints on signal spectra and signal levels, limiting its usefulness in an unacceptable manner to accommodate a relatively small proportion of situations for which such constraints may be necessary.
An object of this invention is to provide a method and apparatus that can permit new communications systems to be added to existing communications paths in a manner that is generally compatible with existing systems where these exist, and that can make optimum use of communications capacity.